flutter_package / flutter_screenutil

fix #186

null safety.1

close #180

Changed window reference

fix #176

I've changed reference of window object as it is recommended in windows docs.

window.dart
/// Please try to avoid statically referencing this and instead use a
/// binding for dependency resolution such as `WidgetsBinding.instance.window`.

binding.dart
/// Each of these other bindings could individually access a [Window] statically,
/// but that would preclude the ability to test these behaviors with a fake 
/// window for verification purposes.  Therefore, [BindingBase] exposes this
/// [Window] for use by other bindings.  A subclass of [BindingBase], such as
/// [TestWidgetsFlutterBinding], can override this accessor to return a
/// different [Window] implementation, such as a [TestWindow].

Init widget

Update screenutil.dart

flutter 1.25 : Error: 'Window' isn't a type.

# Conflicts:
#	README.md
#	README_CN.md

dart doesn't use @param

dart doesn't use @param